1. Field of the Invention
This invention relates to electrical insulation especially useful as primary insulation in high voltage applications. The insulation is a composition of polyethylene combined with an additive which provides resistance to electrical breakdown and another component which mitigates exudation of the additive from the polyethylene.
Electrical breakdown of high voltage insulation, known as dielectric failure, is often initiated at the sites of voids and contaminating particles. Despite extreme care used in making, handling, and extruding polyethylene insulation, voids and contaminants can be introduced in any step prior to final shaping of the insulation to yield an insulated electrical cable. The breakdown of insulation in high voltage applications is known to the trade as electrical "treeing". Treeing is a relatively slow progressive degradation of an insulation composition caused by electron and ion bombardment of the insulation and resulting in the formation of microchannels or tubes having an overall tree-like appearance. Trees are initiated at locations of voids or contamination by the action of ionization (corona) during high voltage surges. Once initiated, trees usually grow, hastened by voltage surges, until such time as dielectric failure occurs.
To overcome the problem of treeing, various additives have been proposed, particularly for use in polyethylene or polyolefins, which additives serve to either prevent formation of trees or serve to delay tree growth. Certain alcohols have been found to be effective additives for delay of tree growth in polyethylene insulation. The alcohols, however, have a tendency to migrate to the surface of the polyethylene. The polyethylene insulation is often made and shipped as small pellets and stored for long periods of time in varying conditions before final extrusion operations to yield insulated electrical cables. The pellets, under certain storage conditions, agglomerate and clump due to the presence of the alcohol additive which has migrated to the surface of the pellets. While the beneficial effect of the alcohol additives in polyethylene insulation is important with regard to delay in tree growth, agglomeration of the polyethylene pellets causes difficulty in material handling which must be corrected.
2. Discussion of the Prior Art
Alcohols of 6 to 24 carbon atoms have been added to polyethylene to yield electrical insulation material having greatly reduced tendency for tree growth. U.S. Pat. No. 4,206,260 issued June 3, 1980, discloses a composition of alcohol and polyethylene and provides evidence of a more than thousand-fold increase in electrical endurance realized by use of an alcohol additive.
Other additives which have been proposed for polyethylene insulation to increase resistance to electrical breakdown include: an inorganic salt of a strong acid with a strong Zwitter-ion compound in U.S. Pat. No. 3,499,791; a ferrocene compound with a substituted quinoline compound in U.S. Pat. No. 3,956,420; silicone fluid in U.S. Pat. No. 3,795,646; and an aromatic ketone in Japanese Pat. No. 14348/75.
U.S. Pat. No. 3,795,646 also provides that polyethylene insulation can be blended with other materials including vinyl acetate, ethyl acrylate, propylene, and butene-1 as well as with copolymers such as ethylene-propylene copolymer. There is caution that the blend must include no less than 50 weight percent ethylene and, more generally, will include 70 to 90 percent.